The long term goal of this project is to improve the quality of life of patients with peripheral artery disease (PAD) using assistive exoskeletons. PAD is a manifestation of systemic atherosclerosis that significantly reduces blood flow to the lower extremities. Veterans with PAD undergo angioplasty, proximal bypass, and distal bypass procedures at rates 10 times higher than the general US male population. Reduced blood flow leads to pain during walking known as claudication. Claudication pain that results from PAD causes patients to walk less and slower, and lose independence compared with their healthy counterparts. Our research has identified functional limitations in the walking patterns of claudicating patients, the most prominent and consistent of which is a deficit of the psterior calf muscles, the ankle plantarflexors, to generate healthy ankle toruqe and power during walking. A properly designed exoskeleton will allow patients with PAD to walk longer without pain or walk the distance needed for completing daily activities with less stress to the affected leg. Before EF can be implemented in patients with PAD, the design must be improved to increase walking performance and actual patient use. As a first step we propose to determine the effect of EF on walking performance, focusing on the how EF contributes to changes in the biological ankle torque and power, energy cost, and calf muscle oxygenation. We will test healthy older adults to ensure and confirm comfort, decrease risk of injury to the foot and calf, and cultivate high potential for device adoption in patients with PAD. The EF will then be evaluated in patients with PAD. Healthy older individuals are selected for this initial phase because like patients with PAD, they produce less push-off torque and power during walking compared with healthy young; making them an appropriate model for PAD limitations. Our specific aims are: Aim 1: To determine the levels of EF assistance force level which produce better walking performance in older subjects. Aim 2: To determine the levels of EF assistance force which produce better subject-reported preference in older subjects. Aim 3: To determine the effect of EF assistance levels on walking performance and subject-reported preference in patients with PAD. Aims 1 and 2 results will guide the assistance levels tested in this aim. 50 healthy older adults for Aims 1 and 2, and 25 patients with PAD for Aim 3 will be recruited from the Nebraska and Western Iowa Veterans Affairs? Medical Center and surrounding Omaha community. Aim 1 will test seven conditions, including five levels of EF assistance, the EF with no spring and one normal walking condition. Lower extremity torque and power will be measured using gait biomechanics on the treadmill for three minutes per condition. Muscle oxygenation will be measured with near infrared spectroscopy of the calf muscles. A metabolic cart will be used to measure energy cost. Aim 2 will thoroughly assess subject-reported preference across levels of EF assistance. Visual analog scales, rate of perceived exertion and pain, and feasibility interviews will provide the comfort and fit, fatigue, and physical activity difficulty of subjects along with the acceptability, demand, and practicality of the EF. Aim 3 will assess walking performance, energy cost, muscle oxygenation, and subject reported preference in the same manner as proposed in Aims 1 and 2, except the treadmill tests will be maximal walking distance tests performed across two difference testing days. We hypothesize there will be an EF assistance level to satisfy subject- reported preference and walking performance goals. This proposal will provide essential fundamental knowledge that will establish EF efficacy and guide prescriptive specifications for EF to be adopted for use by patients with PAD.